Weed control issues account for two of the top five concerns cotton farmers cited in a recent Cotton Incorporated survey, and herbicide resistance is number two on the list.

But even with resistance, cotton producers still have effective and affordable options to control weeds, if they follow a systems approach and do a little homework, says Shane Osborne, Oklahoma State University associate Extension specialist.

Osborne, addressing the inaugural Red River Crops Conference recently in Altus, Okla., said pigweed and horseweed take the top two spots as most difficult weed in Oklahoma cotton. But, unlike other cotton producing states, especially in the Mid-south and Southeast, horseweed takes the top spot, cited by 46 percent of respondents as the worst weed problem. Pigweed was listed by 31 percent. Moringglory came in third at 15 percent and common groundsel was fourth with 8 percent.

Osborne says 82 percent of those recently surveyed said they have resistant pigweed and 78 percent said they had resistant horseweed.

“Cotton farmers currently spend about $32 per acre on herbicides, on average,” Osborne said. “That’s not bad. At current prices that amounts to about 40 pounds of cotton. We are in a lot better shape than other areas of the Cotton Belt.”

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He said identification is important. Farmers should determine if they have pigweed or horseweed. “It’s important to know so we can focus programs and know what to target. We have glyphosate resistant horseweed all over and we began to pick up resistant pigweed a few years ago.”

He said farmers have “a lot of choices to control horseweed,” including pre-plant and burn-down options. But controlling horseweed in season is not an option. “We have no effective herbicides to use in season. Focus on pre-plant; that’s when we can control horseweed.”

Timing is critical. He recommends treating horseweed at the rosette stage. “It makes a big difference in the level of control we get from the rosette stage to early bolt.”

He also cautions producers to look at the 30-day plant-back restriction after using 2,4-D and 21 days (after a 1-inch rain) on dicamba. “And do not use if rainfall for the area averages less than 25 inches per year. Observe the label.”

Control more complicated

With resistant pigweed, control may have become more complicated but producers, “still have options,” Osborne said. “Yellow herbicides still work. A generation of farmers, however, may have grown up with Roundup technology,” and will now have to understand the new environment and go back to some older practices.

“If we use yellow herbicides properly, they are worth the price,” he said. He cited Treflan and Prowl as standbys and good options. Each product has specific requirements, however.

Treflan must be incorporated quickly. “It will not set out in the sun and wait for rain. It breaks down in sunlight. Tillage is the best means of incorporating.”

Prowl “will set out a few weeks,” Osborne said. Tillage incorporation is effective but rainfall or irrigation also do well and can move the material through residue. “Producers may need to increase the rate in heavy residue,” he said.

Both of those products are effective on Palmer amaranth in addition to other annual grass and small-seeded broadleaf weed species, he said, and should be considered in an overall weed control program.

Producers have a lot of pre-emerge herbicide options, used behind the planter and before the cotton and weeds emerge. But growers should do some homework before they select a product. “Know the soil type,” Osborne said. “The possibility of injury exists with some products in some soil types.” Also, rainfall or irrigation will be necessary to activate the products. “We have to have it and sometimes the Southwest struggles to get rain. Since most of our rain comes in April, May and June, we need to focus on the front end of the season to make residual herbicides work here in the Southwest. It’s easier if producers have sprinklers, but not everyone does.”

Over-the-top herbicide options include Roundup and Liberty (on appropriate tolerant varieties), Staple, Warrant, Dual and Prowl. Liberty, Osborne said, is not the preferred product for pigweed here in the Southwest. He also noted the importance of label instructions and timing.

“Each product has label restrictions,” he said. “Know the limitations and the make a plan. Some products have no burn-down effect, only residual.”

Post-emergence materials include Caparol, Cotoran, Karmex, Direx, Layby Pro and Aim. “These may provide burn-down and residual effects but may also injure cotton.” Spray applications that shield the cotton plant may be necessary.

In the new environment of herbicide-resistant weeds, the key to a successful weed control program, Osborne said, is use of residual herbicides. “Those products reduce early season competition and trips across the field while saving fuel and adding different modes of action. Residual herbicides can help to preserve current technology.”

Osborne said when new herbicide crop systems from Monsanto and Dow come out in the next two years or so OSU will look to see where they fit best. “They have shown excellent crop tolerance. And both systems offer remedies to most of our current weed problems. But product stewardship will be important. Scouting weeds and application timing will be important. We also will look at affordability. How much can we afford to pay?”

The systems, which will include either 2, 4-D or dicamba tolerance in a stacked trait variety along with glyphosate and glufosinate, will mean learning new weed control tactics. “Education will be the key,” Osborne said.